Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 955

samples, fermentation media, and even fermentor components can all be sterilized by
placing them inside of the autoclave vessel. Many surgical instruments are sterilized
through the use of very high temperatures, usually in excess of 150 C, to rapidly remove
any contamination. Prepackaged medical instruments are often sterilized through the use
of ethylene oxide or formaldehyde vapors applied over a period of few hours. Biological
contamination of paper machines, water systems, and even pots and pans is often removed
through use of commercially available biocide solutions or material such as ozone or chlo-
rine. All of these are potentially viable methods for the sterilization of fermentation
systems.
Industrial fermentors and associated pipings are designed for in situ steam sterilization
under pressure or SIP. The use of saturated steam is common and advantageous over other
hot fluids. As steam is applied to a vessel or pipe, the hot steam transfers its energy (heat) to
the wall of the pipe or vessel, raising its temperature. The transfer of heat from the steam
decreases the enthalpy of the saturated water vapor, causing it to condense at constant
temperature. This condensation decreases the volume of the water by a factor of 815 for
steam at 121 C. This results in a pumping action (pressure drop when water vapor
condenses), drawing more steam to the areas of greatest consumption, i.e. the areas that
are the coldest. This provides a more even temperature profile and makes the heating
process less likely to result in cold spots and dead legs. As a result, the sterilization is
uniform.
Sterilization of process fluids is dependent on the expectation of probability of successful
fermentations. While relative measures (ratio of surviving cells in the initial cells) have been
used, more valid measure is the absolute sterility or rather the probability of one single cell
surviving in a given fermentation medium. Therefore, the sterility measure is a function of
the medium size. Stringent requirement leads to a longer reduced or dimensionless steriliza-
tion time t S . In a batch sterilization,
t S ¼ k d 0 Z t
0

e E ad

d t lnN 0 (18.27)

RT

The integral can be computed numerically as shown in Table 18.7 for commonly seen
time e temperature profiles. If the temperature is constant, Eqn (18.27) is reduced to

t S ¼ k d t lnN 0 (18.25)

Bacterial spores are known to be more thermal resistant than vegetative forms of yeast,
bacteria, and bacteriophages. Usually, sterilization is designed based on bacterial spores. For
typical spores of B. stearothermophilus, k d0 ¼ 10 37 /min and E ad ¼ 283 kJ/mol.

Larger reactor vessel requires longer sterilization time (or higher temperature) as the
number of unwanted cells, N 0 , is higher in a larger vessel. The same sterilization level can
be maintained with the same t S value. Large-scale bioreactors also place heavy demands
on processes to sterilize fluids entering the bioreactor. A dimensionless sterilization time
of t S ¼

8.236

4.6 yields one unsuccessful fermentation per 100 fermentations, whereas t S ¼ 9.2

yields one unsuccessful fermentation per 10,000 fermentations.